The invention relates to a method of measuring a capacitance, wherein a voltage across the capacitance is supplied to an input of an evaluation circuit and wherein the capacitance is determined from a final voltage value which is produced after discharging of the capacitance from a predetermined starting voltage for a predetermined time period via a predetermined resistance. Furthermore, the invention relates to a circuit arrangement for measuring a capacitance using such a method.
When a capacitance (for example of a capacitor) charged to a predetermined voltage value is discharged via a resistance, then the voltage falls exponentially from the starting value according to the formula:
            U      ⁡              (        t        )              =                  U        0            ·              ⅇ                  -                      t            RC                                ,where U0 is the starting voltage, R is the value of the resistance and C is the capacitance (of the capacitor). If a voltage Umeβ were to be measured after a predetermined time interval T after the start of the discharge via the capacitor, then the capacitance could be determined from the values of Umeβ, T, U0 and R according to the formula:
  C  =                    T        R            ·      ln        ⁢                  ⁢                            U          0                          U                      me            ⁢                                                  ⁢            β                              .      
A determination of the capacitance using this formula would be possible for example if the values of U0 and R could be predetermined as exactly as possible and the most precise measurement possible of Umeβ could take place at a time T which is fixed as exactly as possible. However, known circuit arrangements for determining the capacitance in this way are either very complex in terms of circuitry (and expensive) or are relatively imprecise due to a large number of disruptive influences.
The requirements as to precision are lower if it is not the capacitance itself but only a relative change in the capacitance which is to be determined over a predetermined time period. In this case the precise knowledge of the values of U0 and R is not necessary; these values merely have to be kept constant over the specified time period. Also the exact duration of the time period T is of lesser significance; it is merely necessary that so far as possible the same length of time passes between the start of discharging and the measurement time. In such a case all that is necessary is the most precise measurement possible of the voltage Umeβ.
An example of application in which it is not so much the determination of the absolute capacitance but rather the determination of the relative change in capacitance which is desired, is provided by capacitive proximity sensors for detecting the proximity of an operator's body part to a door handle of a motor vehicle.
The object of the invention is to create an inexpensive and reliable possibility for capacitance measurement for a capacitive proximity sensor.